Dynamic Geometrical Calibration for 3-D Cerebral Angiography

Recently there has been increasing interest in obtaining three-dimensional reconstructions of arterial vessels from multiple planar radiographs (obtained at angles around the object). Interventional angiography is the motivating application behind this research. Different methods have been proposed to acquire the planar data such as a gantry mounted x-ray image intensifier (XRII) or a C-arm mounted XRII. In order to obtain a threedimensional reconstruction from a C-arm mounted XRII the trajectory of the source and detector system must be characterized. We have designed a calibration system that provides the necessary trajectory information using uniquely identifiable markers positioned on a cylinder. This calibration ring is to be placed around the patient's head, and consists of steel balls positioned in a predefined arrangement on a cylindrical acrylic support. The balls are arranged such that calibration can be done from almost any partial view, allowing reconstruction of a region of interest (ROl). Steel balls are placed around an acrylic cylinder, restricted to a band approximately 8.5 mm wide, thereby obscuring only a small fraction of the image. In this case the radiograph includes the region of interest (ROT) as well as a partial view of the calibration ring. This enables us to recover the geometry of X-ray imaging system from each individual frame. We call this process "dynamic calibration" as opposed to "off-line" calibration procedures which try to characterize the motion of C-arm before introducing the patient.